* Add inumber -> inode structure tracking and lookup. This is needed to
ensure that only a single inode structure be used to track multiple
hardlinks to the same place.

* Continue stabilization. Remove modify_tid/delete_tid checks in the
flush code and (for now) only flush along the live path.

Refactor held chains when creating new chains. The creation of a new
chain can move around existing chains, causing the held chain to be
marked deleted.

When a hardlink is consolidated in a parent directory the source chain
used in the duplication is not deleted. Numerous chain->duplink
handling code was assuming that the source was always deleted. Fix
that.